Solid-phase microextraction static headspace sampling

Florez Menendez, J.C. Fernandez Sanchez, M.L. Sanchez Uria, J.E. Fernandez Martinez, E. and Sanz-Medel, A. Static headspace, solid-phase microextraction and headspace solid-phase microextraction for BTEX determination in aqueous samples by gas chromatography. Analytica Chimica Acta 2000,415 (1-2), 9-20. 

Applications The potential of a variety of direct solid sampling methods for in-polymer additive analysis by GC has been reviewed and critically evaluated, in particular, static and dynamic headspace, solid-phase microextraction and thermal desorption [33]. It has been reported that many more products were identified after SPME-GC-MS than after DHS-GC-MS [35], Off-line use of an amino SPE cartridge for sample cleanup and enrichment, followed by TLC, has allowed detection of 11 synthetic colours in beverage products at sub-ppm level [36], SFE-TLC was also used for the analysis of a vitamin oil mixture [16]. 

Miniaturisation of scientific instruments, following on from size reduction of electronic devices, has recently been hyped up in analytical chemistry (Tables 10.19 and 10.20). Typical examples of miniaturisation in sample preparation techniques are micro liquid-liquid extraction (in-vial extraction), ambient static headspace and disc cartridge SPE, solid-phase microextraction (SPME) and stir bar sorptive extraction (SBSE). A main driving force for miniaturisation is the possibility to use MS detection. Also, standard laboratory instrumentation such as GC, HPLC [88] and MS is being miniaturised. Miniaturisation of the LC system is compulsory, because the pressure to decrease solvent usage continues. Quite obviously, compact detectors, such as ECD, LIF, UV (and preferably also MS), are welcome. 

Rancidity measurements are taken by determining the concentration of either the intermediate compounds, or the more stable end products. Peroxide values (PV), thiobarbituric acid (TBA) test, fatty acid analysis, GC volatile analysis, active oxygen method (AOM), and sensory analysis are just some of the methods currently used for this purpose. Peroxide values and TBA tests are two very common rancidity tests however, the actual point of rancidity is discretionary. Determinations based on intermediate compounds (PV) are limited because the same value can represent two different points on the rancidity curve, thus making interpretations difficult. For example, a low PV can represent a sample just starting to become rancid, as well as a sample that has developed an extreme rancid characteristic. The TBA test has similar limitations, in that TBA values are typically quadratic with increasing oxidation. Due to the stability of some of the end-products, headspace GC is a fast and reliable method for oxidation measurement. Headspace techniques include static, dynamic and solid-phase microextraction (SPME) methods. Hexanal, which is the end-product formed from the oxidation of Q-6 unsaturated fatty acids (linoleate), is often found to be a major compound in the volatile profile of food products, and is often chosen as an indicator of oxidation in meals, especially during the early oxidative changes (Shahidi, 1994). 

Solid-phase microextraction (SPME) is a static head-space method similar to the carbon strip method however, it does not require a solvent desorption stage. Volatiles are extracted from the headspace by absorption into an absorbent polymer such as poly-dimethylsiloxane (ASTM method E2154). The absorbent polymer is coated onto a quartz fiber that is housed within a needle similar to a syringe needle. The coated fiber is exposed beyond the tip of the needle in the headspace above the fire debris. As with the carbon strip method, the fiber debris sample can be heated to increase the concentration of volatiles in the headspace. Volatiles are absorbed within the polymer with exposure times for routine screening being within the range 5-15 min. The fiber is retracted within the needle and can then be directly inserted into the injector of a gas chromatograph where the volatiles are thermally desorbed from the polymer onto the column. SPME fibers can be reused but appropriate blanks need to be run to ensure that the fiber is clean. 

General Static Headspace Solid-Phase Microextraction Sampling... 

Figure 3 Typical total ion chromatograms of the headspace volatiles of roasted Ethiopia coffee beans obtained by using static (A) and d5mamic (B) solid-phase microextraction sampling.

Figure 5 Aroma profiles based on gas chromatography olfactometry (GCO) results of the headspace volatiles of Ethiopia coffee beans (roast degree, L23) obtained by using static and dynamic solid-phase microextraction (SPME) sampling methods.

B. Static Headspace Sampling Using Solid-Phase Microextraction... 

The present state of the art in direct solid sampling methods for gas chromatographic analysis of additives encapsulated in polymers is reviewed. Techniques considered are static and dynamic headspace methods, thermal desorption and solid phase microextraction. Applications for each method are discussed. 54 refs. 

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